Four-group zoom lenses for use in television broadcast cameras and in cameras for video recording are known. One known type of four-group zoom lens, for example, includes, in order from the object side; a first group having positive refractive power; a second group having negative refractive power; a third group having positive or negative refractive power; and a fourth group having positive refractive power. In this known type of four-group zoom lens, zooming is performed by moving the second group and the third group along the optical axis.
In addition, there has been a demand in recent years for a small zoom lens that has a high zoom ratio but that is highly efficient as the imaging optics of a television camera or a similar imaging device. This demand has been accompanied by a demand that the small zoom lens provide a wider angle of view. However, in order to achieve a wider angle with a high zoom ratio while maintaining high efficiency, because the peripheral light rays greatly deviate from the optical axis, especially at the wide-angle end, the front (object-side) lens element is required to be larger, which results in increasing the weight of the zoom lens, particularly the weight of the zoom lens at its front (object-side) end. This tends to make the zoom lens difficult to handle both in terms of total weight and poor balance with too much weight at the front (object-side) end of the zoom lens. Additionally, when the angle of view (field angle) of the zoom lens is increased, fluctuations in the angle of view (field angle) and fluctuations in various aberrations increase during focusing of the zoom lens. In particular, fluctuations in the angle of view (field angle) during focusing are especially noticeable at the wide-angle end even if the fluctuations are the same at the wide-angle end and the telephoto end.
In order to alleviate these problems, a so-called inner focus system for zoom lenses has been proposed. In this inner focus system, the lens group on the object side of the zoom lens includes, arranged in order from the object side of the zoom lens along an optical axis; a first lens subgroup having negative refractive power; a second lens subgroup having positive refractive power; and a third lens subgroup having positive refractive power, with focusing being performed by moving the second lens subgroup along the optical axis. This inner focus system for a zoom lens, as compared to other known systems where focusing is performed by moving the entire object-side lens group, has some advantages in keeping the effective aperture of the object-side lens group small, in assisting in reducing the size of the zoom lens, in reducing the force required to drive the lens groups of the zoom lens, and thus in assisting in achieving rapid focusing of the zoom lens. Additionally, the fluctuation in the angle of view (field angle) during focusing is small compared to other focus systems for zoom lenses.
Japanese Laid-Open Patent Application 2001-116993 and Japanese Patent 3376177 disclose zoom lenses using inner focus systems as described above. Japanese Laid-Open Patent Application 2001-116993 discloses a zoom lens with a zoom ratio in the range of 8 to 15, with an f-number of about 1.65 at the wide-angle end, and that is intended to be small and compact as well as highly efficient. Japanese Patent 3376177 discloses a zoom lens with a zoom ratio in the range of 10 to 30, with an f-number in the range of about 1.5 to 1.8 at the wide-angle end, and that is intended to be small and highly efficient. In the zoom lenses described in Japanese Laid-Open Patent Application 2001-116993 and Japanese Patent 3376177, in the first lens group from the object side of the zoom lens, the second lens subgroup from the object side, which is the first lens subgroup from the object side having positive refractive power, is preceded on the object side by a lens component having positive refractive power, which is part of the object-side lens group that has negative refractive power overall.